This invention relates generally to video display systems and in particular is directed to a color video display system having a plurality of memory banks, each storing data directed to a primary color.
Information systems which provide graphic images of geometric figures, alphanumeric characters and various other symbols for display on a screen are known a graphic, or video display, terminals. One type of video display terminal is provided with a cathode ray tube (CRT) having a very low persistence such that it is necessary to add an erasable image memory where the data of the image are stored and may be read repetitively therefrom under the control of a CRT controller in accordance with an operating program stored in a central processor unit (CPU) in order to refresh the displayed image. Because of the need, in generating a cathode ray tube video display, to continually and repetitively refresh the information written on the display, a writable memory such as a random access memory (RAM) having a discrete addressable location for each character to be displayed on the video screen is typically utilized. The video RAM is a read and write memory into which video information is written by the CPU and from which digital data is sequentially read synchronously with the raster scan of the CRT under the control of a CRT controller in accordance with operating instructions stored in the CPU. This video RAM is sequentially addressed to provide a sequence of digital words identifying the alphanumeric characters or other symbols to be visually displayed at successive character locations on the screen.
A color video display typically employs a plurality of refresh memory banks, each representing data corresponding to a primary color. For example, a common arrangement utilizes red, green and blue video memory banks, in which the data stored therein may be selectively accessed in various combinations to provide a full color spectrum. For example, a white symbol may be generated by simultaneously providing information from each of the green, red and blue memory banks to the video display.
In prior art video display systems, each color memory bank is accessed consecutively, or sequentially, by the CPU for writing video data therein. This, of course, imposes upon the CPU additional time during which its operation must be dedicated to providing video information to the video RAM. As a result, the CPU is unable to perform other graphic terminal functions during this period resulting in rather inefficient use of the CPU in the video display system. In addition, the video information displayed on the CRT may only be updated after all color banks are accessed which imposes another limitation upon the quality of the video image and the operating flexibility of the video display terminal.
Accordingly, the present invention is intended to overcome the aforementioned limitations of the prior art by providing an arrangement for simultaneously writing video data into two or more video RAM banks from a CPU in a microprocessor controlled video display system. Each of the video RAM banks represents a primary color, with the video data update time minimized by this simultaneous video RAM access arrangement in providing increased time for other CPU oprations.